In recent years, an occupant protection system for use in a vehicle is developed and is disclosed in, for example, Japanese patent document JP-A-2004-284382. The occupant protection system is used for protecting the occupant of the vehicle when the vehicle collides with an object. The system disclosed in the above-identified document includes a control device and plural collision detection sensors that are interconnected with each other in a daisy chain connection.
Each of the plural collision detection sensors on a network stores a unique ID number for distinguishing itself from other sensors. However, the unique ID number for each of the plural collision detection sensors is assigned to the sensor after installation to the vehicle because of the requirement of production procedure or the like. That is, the unique ID number is assigned to the sensor as an initial configuration setting when the system is initialized by a control device.
The collision detection sensors 5-7 of the occupant protection system are, for example, configured to form a network as shown in an illustration in FIG. 3. That is, the collision detection sensors 5-7 include communication circuits 5a-7a that exchanges signals with a control device 2 (i.e., Airbag ECU in FIG. 3), bus switches 5b-7b that connect/disconnect communication between a preceding device and a subsequent device in the daisy chain connection, acceleration sensors 5c-7c, and RAMs 5d-7d that store unique IDs.
In the initial configuration setting, the control unit 2 conducts the following steps in a sequence. That is, the control unit 2 sequentially outputs ID assign signals to the sensors 5-7 in order from the preceding one toward the subsequent one, and sequentially controls the bus switches 5b-7b to connect to the network. More practically, the control unit 2 outputs the ID assign signal to the first collision detection sensor 5 that is directly connected to the control unit 2, and then the first sensor 5 stores the first ID in the RAM 5d. Further, the bus switch 5b of the first sensor 5 is turned on for connecting the control unit 2 and the second collision detection sensor 6. Then, the control unit 2 repeatedly conducts the same procedure of ID assignment, ID storage and bus switch control on the RAMs 6d, 7d and switches 6b, 7b. In this manner, the all of the sensors 5-7 acquires unique IDs and stores them.
In this configuration, trouble of the bus switches 5b-7b is detected based on a comparison of the number of the sensors in the control unit 2 and the number of the assigned IDs distributed by the control unit 2. That is, when the number of the IDs are identical with the number of the sensors, the bus switches 5b-7b are determined to be correctly operating, and when the number of the IDs are smaller than the number of the sensors, the trouble of the bus switches 5b-7b is detected. The trouble of the switches 5b-7b can be detected in this manner, because, for example, the switch having a short-circuit trouble or an open-circuit trouble can not receive a unique ID properly.
More practically, when the bus switch 5b in FIG. 3 has a trouble at a time of the initial configuration setting, the ID assign signal for the first sensor 5 is also transmitted to the second sensor 6. Therefore, the first and the second sensors 5, 6 have the same ID. After that, the subsequent sensors 7 may have the other ID. However, the number of the sensors stored in the control unit 2 does not become the same number as the number of the IDs for the sensors. That is, the number of the IDs becomes smaller than the number of the sensors 5-7 by 1. As a result, the control unit 2 can detect the trouble of at least one of the bus switches 5b-7b in the plural sensors 5-7.
Furthermore, when bus switch 5b has an open-circuit trouble, the subsequent sensors 6, 7 that is connected on a downstream side of the bus switch 5b can not have the signal. Therefore, even in the case of the open-circuit trouble, the number of the IDs becomes fewer than the number of the sensors. As a result, the control unit 2 can detect an occurrence of the trouble in at least one of the bus switches 5b-7b. 
However, the trouble detection in the above-described manner can only detect the occurrence of the trouble among the bus switches, without having any clue that the trouble is the short-circuit trouble or the open-circuit trouble. Further, the position of the troubled switch cannot be determined. Therefore, operation of the entire occupant protection system is halted when the trouble is detected for preventing malfunction of the occupant protection system.